I. Field of the Invention
This invention related to a fusing apparatus as is commonly used in xerographic copying machines, and more particularly to a fusing apparatus for fusing toner images on a support surface such as a sheet of paper, by applying a plurality of pressure fusing roller strokes to a toned copy sheet in a highly mechanically efficient manner.
II. Description of the Prior Art
In a typical xerographic process a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark and then exposed by being illuminated in an image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination, but retains its electrostatic charge in the dark areas, which areas correspond to the graphic material on the original document. The resulting electrostatic latent image is developed by depositing on the photoconductor a finely divided electrostatically attractable developing material (toner). The toner will normally be attracted to those areas on the photoconductor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This visible image of developing material is then transferred to a support surface, such as plain paper or any other suitable substrate, to become the ultimate copy. Any residual developing material remaining on the photoconductor is removed and the photoconductor is refused as described above for subsequent copies. The toner image that was transferred to the plain paper is then fixed thereto. The fusing process can be either a heat or cold process that employs pressure to fuse the toner particles to the substrate.
One very basic approach to fusing in a xerographic copying machine is the use of the so-called heat and pressure fusing apparatus. Typically, in this apparatus, the paper with the toner image thereon is passed between a pair of opposed and cooperating rollers, at least one of which is heated. Generally, the heated roll is formed of a hollow cylinder having a radiant heater, such as an infrared lamp or a halogen lamp, centrally located within the cylinder to heat the roll, in series with a bimetal thermostat. A typical example of this type of heated fuser roll is illustrated in U.S. Pat. No. 3,637,976. During operation of the fusing apparatus, the paper to which the toner images are electrostatically adhered, is passed through the nip formed between the rolls with the toner image contacting the fuse roll to effect heating of the toner image within the nip. Fusing is enhanced by the second roll or pressure roll as it is commonly called as the result of a biasing force which forces the rolls into engagement. A second basic approach to fusing is a cold pressure fusing process wherein once again paper with the toner image thereon (formed of cold pressure fusible toner particles) is passed through the nip formed between a pair of opposed and cooperating hard surfaced rollers. However, in a cold pressure fusing apparatus, fusing is accomplished by the use of pressure alone.
In general, it would be desirable to increase the operating efficiency of the above described fusing systems. With particular regard to cold pressure fusing systems, not only would it be desirable to increase their operating efficiency, but it would also be advantageous to be able to make these systems simple in design and as inexpensive as possible. Since cold pressure fusing systems fuse by pressure alone, large amounts of pressure must be applied to the rollers which therefor must of necessity be constructed of sufficient strength and size to withstand these large pressures. This of course, tends to make these systems mechanically complex and inefficient, and also expensive, all of which disadvantages are sought to be avoided by the present invention. Examples of some prior art systems which have sought to overcome these difficulties are described in U.S. Pat. Nos. 3,931,793 and 3,988,061.